With regard to inverter boards in recent years, as the frequency of switching operations becomes higher, a switching surge voltage becomes larger due to the inductance in the bus bars in the inverter boards. For this reason, it is important to reduce the switching surge voltage. Employment of so-called sandwich bus bars is effective for a reduction in the inductance. The sandwich bus bars include: a bus bar from which switching current flows into an end of a switching circuit for load control; and a bus bar to which the switching current flows out from the other end of the switching circuit. The two bus bars are set opposite each other with an electrically-insulating material interposed in between.
However, inverter boards are likely to develop trouble, such as short-circuit, due to aging of the sandwich bus bars, and deterioration in insulation performance of the interphase insulating material between the sandwich bus bars. Conventional methods, however, cannot prevent such trouble.
Furthermore, fastening bolts and the like for connecting the bus bars also become loose due to aging, vibration and shock. This leads to a problem of local overheating.
A solution to problems like these is to perform monitoring to prevent sudden trouble inside an electrical appliance board such as an inverter board.
Temperature monitoring apparatuses using optical fibers are used in a method of monitoring temperature abnormalities. Such temperature monitoring apparatuses are put into operation in various fields including tunnel disaster prevention facilities (see PTL 1, for example).
According to PTL 1, one optical fiber cable installed in a tunnel along the lengthwise direction functions in its entire length as a temperature sensor, and collectively measures the temperature distribution in the entire length of the optical fiber cable from one end to the other. Thereby, it is detected whether a fire breaks out.
Although the tunnel disaster prevention facility described in PTL 1 is capable of measuring the temperature distribution along the optical fiber cable in the lengthwise direction, the temperature measurement points are determined depending on the range resolution. For this reason, the tunnel disaster prevention facility are used for large-scale components, and large-scale linear structures typified by tunnels. Such optical fiber temperature monitoring apparatus, as it is, cannot be applied to very narrow spaces inside the appliances.
The applications of optical fiber temperature monitoring apparatuses to narrow spaces are disclosed in PTL 2 and PTL 3.
In an optical fiber temperature sensor described in PTL 2, a portion of an optical fiber cable with a length equal to or longer than that corresponding to the range resolution is wound to measure a local temperature. According to PTL 2, the sensor unit is fixed together with an enhanced thermally-conductive filler using an adhesive, and is disposed in contact with a measurement object.
The temperature measurement precision of the optical fiber cable, however, decreases due to the fixing of the optical fiber cable and the enhanced thermally-conductive filler using the adhesive.